1. Field of the Invention
The invention relates to compositions which can be crosslinked to give elastomers with elimination of alcohols from alkoxysilyl end groups.
2. Description of the Related Art
Single-component silicone rubber mixtures (RTV-1) which can be stored if water is excluded but vulcanized in the presence of water at room temperature to give elastomers are known. They generally comprise a polymeric, mostly linear siloxane, a compound with crosslinking action, which has to contain readily hydrolyzable groups, a plasticizer, which is mostly a methyl-terminated polydimethylsiloxane, and, where appropriate, other additives, such as curing catalysts, pigments, processing aids, and fillers. The vulcanization of the mixtures may take place under acidic conditions, e.g. in the presence of acetoxysilanes, under basic conditions, e.g. by means of aminosilanes, or under neutral conditions, e.g. by way of compounds which have oximo groups or have alkoxy groups. RTV-1 systems which crosslink under neutral conditions are needed especially when the cleavage products arising during the curing of the mixture cannot be permitted to affect the substrate, for example in the jointing of concrete or of metallic materials. In many applications, preference is given to systems which crosslink under neutral conditions because the odor of the elimination products is unpleasant in the acetoxy compositions and amine compositions. The oxime compositions here are increasingly being replaced by the corresponding alkoxy compositions, because of the toxicology of the cleavage products.
For formulating RTV-1 compositions, use is generally made of a polymer having OH end groups. The reaction of the crosslinking agent components with the OH polymer, the substances used for this purpose in the case of the alkoxy compositions being trialkoxyorganosilanes, can take place during the compounding process with the plasticizer and the other fillers. Since these compositions are mostly difficult to produce and are subject to restrictions with regard to mixing specifications (transparent compositions being mostly impossible to obtain), use is generally made of polymers which have been end-capped in advance by alkoxy end groups. Their preparation has been disclosed, by way of example, in EP-B-559 045, and generally takes place via reaction of a hydroxy-terminated polydiorganosiloxane of a particular viscosity with alkoxysilanes in the presence of catalysts. The desired polymer is formed during this condensation process (elimination of alcohol). The preparation of the alkoxy-terminated polymer is then followed by the preparation of the mixture, which can then take place as described above. However, a disadvantage with all of the processes described hitherto is that an adequate degree of conversion of the OH-end-group polymers is achieved only when using methoxysilanes. If the ethoxysilanes known hitherto are used to prepare the polymers, the conversion of the OH groups is incomplete. Because of this, curing of the compositions can occur before preparation of the RTV-1 mixtures has ended, or during the storage of the finished mixtures, and this means that the resultant products have inadequate storage stability.
Another method for preparing alkoxy-crosslinking polymers reacts functionalized polydiorganosiloxanes with an organofunctional alkoxysilane which has a functionality that can be reacted with that of the polydiorganosiloxane. Typical end groups of such polymers may be linear, branched, or, where appropriate, substituted hydroxyalkyl, aminoalkyl, or mercaptoalkyl groups. Suitable functionalities on organofunctional alkoxysilanes, likewise having linear, branched, or, where appropriate, substituted divalent alkyl groups, are isocyanate, epoxy, alkenyl, or anhydride groups. Preference is given here to hydroxy- and aminoalkylpolydiorganosilanes, which may likewise preferably be reacted with isocyanatoalkylsilanes in a simple and quantitative reaction.
EP-A-625 176 describes moisture-cured polymers for applications in antiadhesive coatings. These polymers are prepared by reacting α,ω-aminoalkylpolydimethylsiloxanes with an isocyanatoalkyltrialkoxysilane. The aminoalkylpolydimethylsiloxanes are prepared by equilibrating octamethylcyclotetrasiloxane with bis(3-aminopropyl)tetramethyldisiloxane. U.S. Pat. No. 5,254,657 describes appropriate compounds, including compounds for use in RTV-1 compositions. Here again, the compositions are produced by reacting an isocyanatoalkylalkoxysilane with aminoalkyl-functional polydimethylsiloxanes. The aminoalkylpolydimethylsiloxanes are likewise used in the form of equilibrated systems. A disadvantage of the processes described is that very long reaction times (above 10 hours) are generally needed for equilibration processes to prepare aminoalkylpolydiorganosiloxanes, mostly giving inhomogeneous polymers. A portion of the cyclosiloxanes mostly remains unreacted here, and moreover a portion of the added bis(3-aminopropyl)tetramethyldisiloxane does not become incorporated into the polymer. The resultant polymers therefore have an inhomogeneous and broad molecular weight distribution. The high residual content of from 10 to 20% of octamethylcyclotetrasiloxane in the equilibrated system generally has to be removed in vacuo by a complicated process. U.S. Pat. No. 5,254,657 omits this purification process, but this method is problematic in relation to the mechanical properties of these compositions and the bleed-out of low-molecular-weight constituents after application.
Finally, the catalyst in the equilibrated system also has to be adequately deactivated to prevent premature vulcanization of the polymers, for example during the incorporation of fillers in the compositions. Because the reaction conditions during the equilibration process and the deactivation of the catalyst are extreme (high-temperature heating of the polymer mostly being used), compositions of this type mostly have an amine-like odor, together with a yellow color.
U.S. Pat. No. 3,146,250 describes another process, not hitherto used industrially, for preparing aminoalkylpolydimethylsiloxanes, starting from specific cyclic silanes of the general formula (1), which can react with HO—Si groups at the end of a silicone chain.
R′ here is a carbon chain having at least 3 and at most 6 carbon atoms, R″ is a hydrocarbon radical, and the radical R′″ on the nitrogen is either hydrogen, a hydrocarbon radical, or an organosilyl radical of the general formula (halogen-R″″—)Y2Si—, where Y and R″″ are hydrocarbon radicals. If the radical R′″ is hydrogen, the result is an unsubstituted cyclic silazane which can be used to functionalize hydroxy-terminated silanols. The disadvantage of these unsubstituted cyclic silazanes is, however, that they can be synthesized only in very poor yields or from expensive starting materials. In these syntheses use is frequently made of the highly toxic compound allylamine, the use of which requires particularly stringent safety measures. The synthesis of N-substituted silazanes is of greater interest, the preparation of these frequently giving better yields. However, if use is made of the N-silyl-substituted silazane described in U.S. Pat. No. 3,146,250, a reaction with hydroxy-terminated siloxanes gives undesired by-products, such as those listed in the formula diagram below:
Y and X here are methyl, for example, and R is propyl, and Hal is halogen, for example chlorine. The respective proportions of the starting materials are derivable from simple probability calculations.
If use is made of the N-substituted silazanes of the general formula (2)
described in DE-A-3546376, where R is, by way of example, a propyl group, the result is likewise considerable amounts of undesired by-products, by analogy with the reaction described above (see above formula diagram).